Conventionally, an electric pump device has been known as a device for supplying oil to a hydraulic device such as a continuously variable transmission (CVT) or a power steering device of a vehicle (see, for example, Patent Document 1).
More specifically, an electric pump device Q, for example, is known. As shown in FIG. 7, the electric pump device Q has an oil pump 105, which is driven by a brushless motor 106 to apply hydraulic pressure to a continuously variable transmission T, which is a hydraulic device. The electric pump device Q is employed in combination with a mechanical oil pump 103, which is driven by an engine 104. When the mechanical oil pump 103 cannot supply a sufficient amount of oil to the continuously variable transmission T, the electric pump device Q supplies oil to the continuously variable transmission T in an assisting manner.
The electric pump device Q includes the oil pump 105, which supplies oil to the continuously variable transmission T, the brushless motor 106 for driving the oil pump 105, and a controller 107 that controls the brushless motor 106. The electric pump device Q further includes a relief valve 108 that prevents loss of synchronism of the brushless motor 106.
When the oil pump 105 supplies an excessive amount of oil from an inlet portion 151 to an outlet portion 152 of the oil pump 105, the relief valve 108 returns the oil from the outlet portion 152 to the inlet portion 151 of the oil pump 105. Accordingly, even if the hydraulic pressure at the outlet portion 152 of the oil pump 105 is raised through excessive oil supply, the electric pump device Q having the relief valve 108 lowers the hydraulic pressure at the outlet portion 152 so as to prevent the brushless motor 106 from receiving excessive load. This prevents loss of synchronism of the brushless motor 106.